Added testcase for ListFields with just pks (#361)
Added testcase for ListFields with just pks (#361)

RussellLuo/mongoengine,cogniteev/mongoengine,khallaghi/mongoengine,gmacon/mongoengine,MongoEngine/mongoengine,muhammad-ammar/mongoengine,starsirius/mongoengine,eli-b/mongoengine,sourcelair/mongoengine,elephanter/mongoengine,nicolascine/mongoengine,Catstyle/mongoengine,reallistic/mongoengine,WingGao/mongoengine,ProgressivePlanning/mongoengine,Davidrjx/mongoengine,iandd0824/mongoengine,MakerReduxCorp/mongoengine,Davidrjx/mongoengine,yarneo/mongoengine,nathanial/mongoengine,9nix00/mongoengine,noirbizarre/mongoengine,losintikfos/mongoengine,Multiposting/mongoengine,ms5/mongoengine,larsbutler/mongoengine,MakerReduxCorp/mongoengine,touilleMan/mongoengine,ConsumerPhysics/mongoengine,prijindal/mongoengine,DavidBord/mongoengine,MRigal/mongoengine,jimmyshen/mongoengine,cogniteev/mongoengine,gavincabbage/mongoengine,closeio/mongoengine,blaggacao/mongoengine